The present invention relates to articles having high temperature coatings of the MCrAlY type.
It is well known that the family of high temperature, oxidation-corrosion resistant coatings commonly referred to as MCrAlY coatings can markedly extend the service life of gas turbine blades, vanes and like components; for example, see U.S. Pat. Nos. to Evans et al 3,676,085; Goward et al 3,754,903 and Talboom Jr. et al 3,542,530, all of which are of common assignee with the present invention. The MCrAlY coatings are referred to as overlay coatings, denoting the fact that they are deposited on the substrate as an alloy and act substantially independently of the substrate in providing oxidation-corrosion protection.
In the past, these coatings have been applied to superalloy substrates by vapor deposition, sputtering, and plasma spraying techniques.
Of the three, sputtering is of the least current interest because of its relatively low rate of deposition. Vapor deposition coatings have received the most attention because of the cleanliness of the vacuum-applied coating and the control which is possible. Vapor deposited coatings are comprised of a plurality of abutting columnar grains typically oriented normal to the surface of the superalloy substrate. However, the vapor deposited coating composition choice is limited to elements which are compatibly vaporized.
Plasma spraying appears to offer the greatest versatility in manufacturing operations: equipment cost is comparatively low, variation in thickness from point to point is readily achievable, and coating composition can be readily altered by change in powder. However, in plasma spraying MCrAlY coating alloys on superalloy substrates, less than satisfactory results have been experienced due to the development of interconnected, as well as isolated pores, voids and like defects in the coating, some of which extend to and penetrate the outer or free surface of the coating. It has been observed that such defects adversely affect the oxidation-corrosion resistance of MCrAlY coatings, for example, as compared to that of similar vapor deposited coatings.
Various improvements in MCrAlY coatings have been disclosed previously in U.S. Patents. In vapor deposited MCrAlY type coating, Walker et al U.S. Pat. No. 3,873,347, indicates that an overcoat of aluminum increases the corrosion resistance. The aluminum was applied by the conventional pack cementation process at 600.degree.-1000.degree. C. Some specimens were post heat treated at 1080.degree. C. It was stated that laboratory tests showed improved corrosion resistance, although this is somewhat at variance with the conventional wisdom now that aluminum enrichment provides mostly improved oxidation rather than corrosion resistance. By weight percent, Walker's broadest MCrAlY is 14-35 Cr, 4-20 Al, 0.1-34, and the surface is aluminized to a depth of about 1-2 mils (0.025-0.05 mm).
Bessen U.S. Pat. No. 3,961,098 discloses a coating produced by plasma spraying according to a certain specialized technique, followed by subsequent infiltration into the surface of the sprayed coating to fill the voids. Unexpectedly aluminum and chromium are reported to be diffused through the coating, even to the point of obtaining some negative concentration gradients with respect to the surface, as shown in FIGS. 4-6 of the patent. Further, the MCrAlY plasma coating of Bessen is said to not be a conventional one wherein the particles are melted, but instead to be comprised of "heated rather than molten particles in order to enhance retention of deformation . . .". Subsequently, when aluminum or chromium are interdiffused, the MCrAlY recrystallizes. Aluminum of 8-20% by weight and chromium contents of 20-30% are taught in Bessen.
In our U.S. Pat. No. 4,152,223 we disclose and claim an improved method of obtaining plasma sprayed MCrAlY coatings, wherein the plasma sprayed coating, having been deposited on the substrate, is provided with a metallic envelope and is then hot isostatically pressed (HIP) to seal the defects and interdiffuse the envelope with the plasma spray coating.